A measuring tool that is commonly used in waveform analysis is the oscilloscope, which provides a visual representation of a waveform. A conventional real time oscilloscope provides a continuous but ephemeral representation of a waveform. On the other hand, a digital storage oscilloscope, which samples the waveform and writes the sampled magnitudes into memory, can provide a more enduring display of the waveform by reading the stored magnitude information from memory, converting the stored magnitude information to analog form and using the resulting analog signal to drive the vertical deflection amplifier of the oscilloscope. The samples are taken at predetermined times relative to the time at which the signal magnitude passes through a selected trigger level. Time information, for driving the horizontal deflection amplifier, is derived from the clock signals used for reading the magnitude information from memory.
The conventional digital storage oscilloscope can be used to provide a representation of the waveform of a repetitive signal having components at frequencies that are higher than the sampling frequency, because by taking samples over a large number of repetitions of the signal a sufficient volume of data can be accumulated to provide an accurate display of a single repetition. Known digital storage oscilloscopes also provide means for calculating waveform parameters, such as maximum and minimum magnitude.
An equivalent time sampling system can provide information relating to input signals having frequency components that are higher than the sampling rate. However, the amount of information that can be obtained is limited if the waveform of the input signal is not identical from repetition to repetition. If the waveform of the input signal is not identical from repetition to repetition, the signal has jitter. A displayed waveform has jitter, even if the waveform of the input signal is identical from repetition to repetition, if the trigger level, which determines the time origin for sampling purposes, varies from repetition to repetition.
FIG. 1 illustrates the display that might be provided by a digital storage oscilloscope in response to a repetitive analog signal pulse that has variations in the time at which the first and second transitions occur relative to the trigger point. It will be seen that portions of the waveform representing the first and second transitions are thickened (represented by diagonal hatching in FIG. 1), reducing the accuracy of measurements made on the waveform. It is desirable that the presence of jitter be identified and that the nature of the jitter be evaluated in order to identify the source of the jitter and to compensate for jitter. For example, if a particular feature of the input signal waveform, which should nominally occur at the same time relative to the trigger point, occurs over a band of time values, and the distribution of occurrences has a bell-shaped or Gaussian distribution, this implies that the source of jitter is noise, whereas if the distribution has two or more distinct peaks the implication is that the source of jitter is an error in transforming an analog value into a digital value or that the jitter is non-Gaussian jitter caused by harmonic noise.